The cleanup of acid gasses, such as CO2, from natural gas has been an extensively practiced technology. The industrial removal of CO2 from this natural gas dates back to the 1930's. While several technologies exist for the removal of acid gasses one of the most commonly employed practices is the use of aqueous amines. Post-combustion CO2 capture is a newer area of interest, but the principles remain the same. The overall process is depicted in FIG. 1. An aqueous amine solution is circulated between the absorber 10 and stripper 12. The flue gas, containing CO2, enters the bottom of the absorber 10 where it contacts the aqueous amine absorbent removing it from the gas stream. The liquid solution, CO2 rich amine solution, is then passed through a heat exchanger 14 to improve efficiency before being heated to a higher temperature in the stripper 12. The stripper 12 removes the CO2 as a gas from the amine solution to produce a lean, or CO2 deficient solution. The lean solution is returned to the absorber 10 by way of the heat exchanger 14 to repeat the process. The CO2 removed from the gas stream is then available for subsequent use such as Enhanced Oil Recovery (EOR), utilization in downstream products (polymers or chemicals), or for sequestration.
EOR currently represents the use of 54 MMT/y of CO2 and is projected to grow by about 50% by the year 2020. Currently, the bulk of the CO2 for EOR is naturally occurring with only about 20% of CO2 used from anthropogenic sources. However, the key limitation to further deployment of EOR in the US is the supply of CO2. There is increased motivation and interest in the use of post-combustion CO2 to expand this market and debottleneck the CO2 supply.
There are also other smaller potential post-combustion CO2 capture markets. Examples include the production of food grade CO2 (beverage carbonation) and sodium carbonate. There is also growing interest in the downstream utilization of CO2 into value added products using post-combustion captured CO2. The products include polycarbonates, urea, carboxylic acids, etc. These markets represent a shorter term market opportunity for the described process.
In the longer term CO2 capture and sequestration (CCS) represents an enormous potential market for the described process. At present, the energy sector is responsible for about three-fourths of the anthropogenic carbon dioxide emissions. Over the past 15 years, economic activity in the U.S. has increased by about 50%; total U.S. electricity demand has increased 30% over the same period. In the coming years, the surge in the U.S. demand for electric power shows no signs of abating. Economic activity in the U.S. is projected to expand 49% by 2020. Accordingly, in the same period, the demand for electricity is projected to increase by another 30%. It is projected that fossil fuel combustion will still represent a large portion of the total US energy supply. The regulation or limitation on CO2 emissions from fossil fuel combustion power plants would necessitate the large scale, commercial deployment of CO2 capture and sequestration solutions as described above.
The actual process for CO2 capture depicted in FIG. 1 is complicated by numerous factors, including the presence of oxidative contaminants in the flue gas in addition to the targeted CO2. The flue gas contains various potential oxidants (O2, SOx, and NOx) which can degrade the solvent. During the high temperature regeneration process, the components dissolved in the solution from the flue gas in the absorber at relatively low temperature can further degrade the solvent. Furthermore, the solvent can undergo thermal degradation to yield more impurities potentially subject to degradation.
One of the critical degradation paths is the reaction of NOx with amine solvents to generate nitrosamines and nitramines. These compounds are of particular interest due to their potential for secondary environmental impact if released to the environment (as a gas or in liquid phase). The utilization of process methods to reduce nitrosation products is thus valuable for amine-based CO2 capture.
Chemical additives provide one possible route to reducing nitrosation. The additive or inhibitor presumably works by reacting with the active radical species to prevent the reaction with the amine. Due to the low concentration of active radical species in solution the compound is needed in only small concentration to effectively inhibit the nitrosation reaction.